Recent advances have led to the development of clinical tests that provide site-of-lesion information about vestibular (inner ear balance) function in patients suffering from dizziness or imbalance. Specifically, the vestibular evoked myogenic potential (VEMP) supplements the vestibular test battery by providing information about otolith organ function (a type of vestibular sensory organ). Although VEMPs are becoming more widely used in vestibular clinics throughout the world, the clinical significance of otolith organ dysfunction is unclear. The purpose of this study is to determine the consequences of otolith organ dysfunction on postural stability. These findings may lead to the development of novel approaches for patients with otolith organ dysfunction. The vestibular system is comprised of two types of sensory organs (semicircular canals and otolith organs) with unique contributions to balance. Loss of vestibular function can occur in one or both labyrinths, in one or both branches of the vestibular nerve, and in one or more vestibular sensory organs. Symptoms of vestibular loss include postural instability, visual blurring, and dizziness and/or imbalance. Moreover, the incidence of falls is greater in individuals with vestibular loss than in healthy individuals of the same age. The incidence of dizziness and imbalance increases in populations relevant to VA healthcare: older adults and individuals with traumatic brain injury (TBI). Preliminary data suggest that otolith dysfunction is prevalent in Veterans with TBI. Most research examining the effect of peripheral vestibular dysfunction on postural stability have used measurements of horizontal semicircular canals (hSCC) to determine vestibular loss (e.g., Herdman et al., 2000), because conventional vestibular clinical assessment has been limited to caloric or rotational testing (tests of hSCC/superior vestibular nerve function). Thus, it is unclear how dysfunction in other components of the vestibular system contributes to postural instability, and importantly, the incidence of falls. The use of tests of otolith function in addition to the tests of hSCC function provide the ability to differentiate vestibular end organ, and better explain the effect of peripheal vestibular loss on postural instability. Vestibular rehabilitation includes gaze stability exercise based on principles of adaptation of the vestibulo-ocular reflex (or semicircular canal); therefore this study has important implications for rehabilitation of patients with otolith dysfunction. The International Classification of Functioning, Disability, and Health (ICF) model will provide an organizational structure for the study to examine the complex relationships between Health Condition (in the proposed study, this refers to the specific vestibular groups: canal dysfunction otolith dysfunction, canal + otolith dysfunction), the components of health, and the context. The specific domains of the ICF include: body function and structure (body level); activity (individual level) and participation (societal level). Importantly, the ICF model considers both environmental and personal factors that impact function and disability. The specific aims are designed to determine the effect of otolith dysfunction on postural stability at the levels of impairment (Specific Aim 1), activity and participation (Specific Aim 2), and determine the interaction of age and otolith dysfunction on postural stability (Specific Aim 3). To address these aims, 5 groups of Veterans based on vestibular site-of-lesion tests (3 vestibular groups and 2 control groups) will be enrolled in the study (n = 124). Three vestibular groups will include Veterans complaining of dizziness and/or imbalance with: (1) horizontal semicircular canal (hSCC) dysfunction only (Canal Only), (2) otolith dysfunction only (Otolith Only), and (3) hSCC and otolith dysfunction (Canal+Otolith). The control groups will include (1) Veterans complaining of dizziness and/or imbalance with normal hSCC and otolith function (Dizzy Control) and (2) age- and gender- matched Veterans with no complaints of dizziness and/or imbalance and normal hSCC and otolith function (Healthy Control). To address the specific aims, balance measures, gait and mobility measures and self-report questionnaires will be compared between vestibular groups and across a wide age range.